The disclosure relates to poly(arylene ether)/polyamide compositions containing mineral fillers, and a method of making these compositions.
Poly(arylene ether)-polyamide blends have established themselves as highly attractive materials from which an extremely diverse range of products can be molded. The blends exhibit the most desirable properties of each material, i.e., outstanding heat resistance and dimensional stability from the poly(arylene ether), and excellent strength and chemical resistance from the polyamide. Exemplary disclosures of such compositions are found in U.S. Pat. No. 6,469,093 (Koevoets et al); U.S. Pat. No. 4,873,276 (Fujii et al); U.S. Pat. No. 4,659,760 (Van der Meer), U.S. Pat. No. 4,732,938 (Grant et al), and U.S. Pat. No. 4,315,086 (Ueno et al).
The properties of the poly(arylene ether)-polyamide blends are often enhanced by the addition of compatibilizing agents for the base resins; flow modifiers, impact modifiers, and fillers. The blends are very popular for use in molding a variety of automobile parts, e.g., body panels, trunk lids, hoods, and many smaller components, such as fuel filler doors and mirror housings. Replacement of metal automobile components with plastic parts has many advantages, such as weight reduction and the elimination of corrosion problems. However, in many automotive applications, the plastic component must appear identical to adjacent metal components, in terms of color, texture, and overall appearance.
An electrostatic painting operation is often used to apply paint to plastic and metal components in automotive assembly operations. Since such a painting operation requires that the plastic be sufficiently conductive, conductivity additives must be incorporated into the plastic. Moreover, since the paint is usually baked onto the panel at elevated temperatures (e.g., greater than 180° C.), the plastic component must be able to withstand such temperatures, without degradation or warpage.
Many types of poly(arylene ether)-polyamide blends have the heat resistance needed for the painting and baking operations. Moreover, the blends have been adequately formulated to provide the necessary conductivity for electrostatic-painting, although the required additives can be expensive and difficult to handle. Their presence can also adversely affect other properties, such as ductility and surface appearance.
Technologists have become very skilled in formulating poly(arylene ether)-polyamide blends which balance a number of required properties. However problems can still occur for a variety of end use applications and manufacturing conditions. For example, it can be very difficult to ensure that a molded part has the low coefficient of thermal expansion (CTE) required in many instances. In the case of fuel filler doors and other parts which may be closely positioned between metal components, a low CTE is often critical for preventing expansion and “sticking” of the parts. These close tolerances can be very difficult to consistently maintain when the automobile is repeatedly exposed to sunlight and high outdoor temperatures during use.
Moreover, both consumers and dealers of automobiles (as well as manufacturers) place a great emphasis on the overall, painted appearance of a car or truck. A molded, painted plastic part often must have a “Class A” surface, or an appearance approaching Class A quality. As noted in U.S. Pat. No. 5,965,655 (Mordecai et al), there are various definitions of “Class A”. In general, a Class A surface as used herein is meant to describe a painted plastic surface which is substantially identical to an adjacent sheet-metal surface, in terms of texture (i.e., smoothness) and color. Furthermore, the surface should be generally free of various defects, such as splay and paint popping.
Some of the commercially-available poly(arylene ether)-polyamide compositions for electrostatic-painted applications employ talcum (talc) as a filler. While these products usually exhibit a good balance of dimensional stability, rigidity, heat resistance, and impact strength, they may exhibit some drawbacks under certain conditions. For example, when the compositions are molded, the presence of talc can lead to splay. Splay is often manifested as streaking or pale marks on the molded article—usually near the mold gate. Moreover, when the articles are painted, other surface defects sometimes occur, such as paint popping. These problems seem to be more prevalent when talcum is present along with the conductivity additive.
The undesirable surface defects can sometimes be minimized or eliminated by careful adjustment and maintenance of molding conditions, such as temperature and cycle time. However, the processing window can be very narrow, i.e., small deviations from the pre-set molding conditions bring on the occurrence of the defects. This narrow processing window is a considerable disadvantage in an industrial setting, leading to losses in processing time and efficiency.
Organic clays such as kaolin have been proposed as an alternative to talc and has been shown to have good surface properties after painting. However, currently available compositions comprising organic clays do not have the mechanical properties required for some applications. In particular there is a desire for compositions having a lower coefficient of thermal expansion (CTE).
Despite numerous attempts to produce compositions having a combination of mechanical properties and surface appearance there remains an ongoing need for compositions having improved impact strength and surface appearance as well as methods of making these compositions.